1. Field of the Invention
The present invention relates to an X-ray flat panel detector used in a medical X-ray diagnosis apparatus or the like and, more particularly, to an X-ray flat panel detector which uses a thin-film transistor as a switching element in each of two-dimensionally arrayed X-ray detection pixels.
2. Description of the Related Art
In recent years, X-ray flat panel detectors for electrically detecting an X-ray image have been developed as X-ray diagnosis apparatuses. In such an X-ray flat panel detector, X-ray photosensitive pixels which generate signal charges when being irradiated with X-rays are two-dimensionally arrayed. Signal charges generated by these photosensitive pixels are stored in signal charge storing sections and read out by thin-film transistors serving as switching elements.
FIG. 1 shows the circuit arrangement of one pixel of this X-ray flat panel detector. As shown in FIG. 1, the X-ray flat panel detector has a plurality of pixels 101 that are arranged in an array. Each pixel 101 is constituted by a thin-film transistor (to be referred to as a TFT hereinafter) 102 made of amorphous silicon (to be referred to as a-Si hereinafter) and used as a switching element, an X-ray charge conversion film 103 made of Se, a pixel capacitor (to be referred to as a Cst hereinafter) 104, and a protective TFT 111. The Cst 104 is connected to a Cst bias line 105.
The switching TFT 102 has a gate connected to a scanning line 107 and a source connected to a signal line 108. The TFT 102 is ON/OFF-controlled by a scanning line driving circuit 109. A terminal of the signal line 108 is connected to a signal detection amplifier 110. The source and gate of the protective TFT 111 are connected to the drain of the switching TFT 102. The drain of the protective TFT 111 is connected to a power supply 113 through a bias line 112. Note that the protective TFT 111 is sometimes omitted.
When X-rays become incident, they are converted into charges by the X-ray charge conversion film 103. The charges are stored in the Cst 104. When the scanning line 107 is driven by the scanning line driving circuit 109 to turn on the switching TFTs 102 of one row connected to one scanning line 107, the stored signal charges are transferred to the amplifier 110 side through the signal line 108. With change-over switches (not shown), the charges are input to the amplifier 110 for each pixel and converted into a dot sequential signal that is displayable on a CRT or the like. The charge amount and the output amplitude of the amplifier 110 change depending on the amount of x-ray light incident on the pixels 101. To prevent extra charges from being stored in the Cst 104, charges more than the bias voltage are removed from the bias line 112 by the protective TFT 111.
However, the presence of a protective circuit such as the protective TFT 111 poses the following problems. First, since the pixel circuit is complicated, the yield of non-defective TFT array boards decreases. Second, when the number of pixels is increased and the pixel size is reduced to obtain a higher resolution, it becomes difficult to form interconnections from the protective TFTs 111, switching TFTs 102, and the like at a sufficiently small pitch. Third, since the occupation area of the protective TFT 111 in each size-reduced pixel is large, no sufficient area can be ensured for the Cst 104.
In addition, when the number of pixels is increased and the pixel size is reduced to obtain a higher resolution, the following problems have occurred. To externally connect a TAB having a driving circuit for driving switching TFTs and an LSI for reading a signal outside the detection pixel area, these interconnections must be connected to the TFT array board on which the switching TFT 102, x-ray charge conversion film 103, Cst 104, and protective TFT 111 are arranged. At this time, if the pixel size is reduced, it becomes difficult to connect the interconnections at a sufficiently small pitch. In addition, since the ratio of the TFT area in the small pixel is too high, the signal storage capacitor cannot have a sufficient area. Furthermore, when the number of pixels increases, the signal read time per line shortens. However, in the TFT formed from a-Si, a signal cannot be sufficiently read in the short address time. For this reason, in the conventional X-ray flat panel detector and, more particularly, an X-ray flat panel detector using a-Si TFTs, when the pixel size is reduced and the number of pixels is increased to obtain a higher resolution, no satisfactory detection image can be obtained because of shortage in TFT driving capability.
For an X-ray detector for medical use, it is important to do diagnosis using an X-ray dose as low as possible to suppress the influence on the body. To do this, it is important to reduce the OFF current of a switching TFT to make it possible to detect a small amount of charges generated at a low X-ray dose. On the other hand, a satisfactory image is necessary for accurate diagnosis. In this case, it is necessary to sense an image with a high S/N ratio at a high X-ray dose. Hence, it is preferable to sense an image at an X-ray dose ranging from low level to high level. For this purpose, a switching TFT must normally operate even at a high pixel voltage corresponding to a large amount of charges. In addition, the OFF current of the switching TFT must be reduced. Many switching TFTs used in such an X-ray flat panel detector form defects by X-ray irradiation, and their characteristic degrades. The switching TFT must have a TFT characteristic that allows the TFT to operate even after degradation.